In the design of pulse doppler radar fuzes it is necessary to meet and solve the problems surrounding the provision of means by which the fuze can operate normally in the presence of ground clutter and various types of electronic counter-measures. For example, it is desirable to have a fuze which will operate normally in the presence of ground clutter, will not be fired when subjected to side lobe jamming, will function normally in a benign electronic counter-measures environment and will fire when a strong jamming signal is directed into the main beam. There have been numerous attempts previously to solve each of these problems separately. The circuit designs resulting from these solutions, however, have proven generally to be incompatible making it virtually impossible to obtain a fuze which will operate normally when subjected to all of the aforementioned difficulties. Further, previous solutions offered to meet these problems, especially those directed to electronic counter-measures, have tended to degrade the performance in reliability of the fuzes normal target sensing operation in a benign environment.
The principal solution offered previously to meet the problem of clutter and incoherent electronic counter-measures has been the use of frequency discrimination. That is, previous fuzes have provided two receiver channels of different bandpass frequencies which would separate the desired from the undesired signals reaching the fuze receiver. But, as will be discussed further hereinbelow, this has proven to be an incomplete solution.
The most serious difficulties occur when the fuze is required to function properly in the presence of severe electronic counter-measures interference. The most probable threat is considered to be the self-screening jammer, that is, where the target aircraft carries its own source of jamming signal. The other type of jamming is provided by the so-called stand-off jammer which is a ground based or airborne station which provides a jamming signal which shields the target aircraft. The threat from the self-screening jammer, however, is considered most probable since the source of jamming radiation is aboard the target aircraft and there is no question as to relative range and location of the jammer and the target such as exists for the stand-off jammer. The effectiveness of the airborne stand-off or escort jammer would depend in large measure on the range and position of the jamming aircraft relative to the fuze and the fuzes antenna pattern for the brief time it is active during the target intercept phase. Therefore, while the fuze design must attempt to minimize the susceptibility to the stand-off jammer, knowledge of the tactical air-borne electronic counter-measures environment during the intercept in which the fuze is to be used is required to actually evaluate the vulnerability of the fuze to stand-off jamming under the particular circumstances. Lacking this information it will be assumed herein that the stand-off jammer is a real potential threat but second in importance to the self-screening jammer which constitutes a threat more real than potential. Obviously, it would be desirable that any protective means designed to shield a proximity fuze from electronic counter-measures jamming in an environment where the different types of jamming are present be able to discriminate these various different types. Unfortunately, the simple protective devices which cause a fuze to fire or not to fire in particular jamming situations do not suffice for those situations in which more sophisticated jamming techniques are used. That is, self-screening jamming and stand-off jamming impose conflicting requirements on any protective circuitry with the result that many of the simple devices used heretofore will not operate properly in such environments.
An electronic counter counter-measure remedy previously proposed for use in an environment where both stand-off jamming and self-screening jamming are present was to add a separate receiver channel and antenna to guard the fuze antenna side lobes. The guard channel would be a broad beam receiving channel adjusted to exceed the fuze antenna side lobe gain but less than the fuze main beam gain which would subtract from the normal fuze receiver output. The effect of this device would be to provide a geometry discrimination by forcing the self-screening jammer into the fuze main beam to cause the fuze to function. That is, it would provide a situation in which a jamming signal in the main beam would cause the fuze to fire but a similar signal in the side lobes would be prevented from causing the fuze to fire. But, this technique proved to be an incomplete solution to the problem. While the guard channel circuitry has merit in defeating the self-screening jammer, it could not be properly integrated into a fuze system which would also provide satisfactory rejection of ground clutter. The use of this electronic counter countermeasure technique presented other difficulties including a lack of any protection against the stand-off jammer in the main beam; a lack of capability of seeing targets in the forward side lobes, hence, the inability to fuze the collision course target; possibility of a fuze dud due to a stand-off jammer in the side lobe nulls; and the rather critical adjustments required to set the proper relationship between the guard beam, the main beam, and the side lobe gain and the possible lack of sensitivity if this relationship were disturbed for any reason.
Because the prior art systems discussed above generally require a relatively high pulse repetition frequency, dudding due to ground clutter can conceivably occur over an uncomfortably large portion of possible target engagement altitudes. This is due to the fact that the density of altitude clutter zones is proportional to the pulse repetition frequency.
It is therefore an object of this invention to provide a means which will allow a pulse doppler radar fuze to meet the conflicting requirements of being able to discriminate against ground clutter and side lobe jamming and to fire on a strong jamming signal in the main beam while being able to function normally in a benign electronic counter-measure environment.
It is a further object of this invention to provide a means which will allow a pulse doppler radar fuze which will be substantially unaffected by ground clutter.
Another object of this invention is to provide a means which will allow a pulse doppler radar fuze to meet the above objects while maintaining adequate sensitivity and sufficient reliability.